In the prior art, a steam box is known which is used in connection with a board or paper machine and through which hot steam is passed into connection with a board or paper web in order to reduce the viscosity of the water contained in the web and thereby to improve the removal of water from the web, i.e., increase the dry solids in the web. Steam boxes are used for the above-mentioned purpose, for instance, in wire and press sections of a board or paper machine.
One particular problem in the steam boxes used in wire sections is the blowing-out of non-condensing steam from the active area of the steam box which wets the structures in the press section and even in the entire paper machine hall. This mist cloud of non-condensing steam produced by the blowing-out of steam from the steam box causes, in the worst cases, even water to drip from the ceiling of the machine hall. Steam blows out mainly from the trailing edge of the steam box and to some extent from the sides of the steam box.
In view of this blowing phenomenon, the capacity of the whole steam box cannot be used efficiently for increasing dry solids in the web because in that case, the formation of mist is intense and the structures of the machine become too wet. One solution to this problem known from the prior art is to provide for suction on the trailing side of the steam box. However, this has not made it possible to sufficiently eliminate the spreading of steam as mist to surrounding structures and the machine hall, and thus it has nevertheless been necessary to operate the steam box at reduced capacity because of the blowing-out of steam from the active area of the steam box.
As another solution to this problem, attempts have also been made to arrange general exhausts in the area of the wire section. However, this has not made it possible to sufficiently prevent the spreading of steam and its condensation on structures and the resulting dripping of water therefrom.
Another serious problem in the steam box of a wire section is keeping the steam box clean in the area of edge cutting, i.e., in the vicinity of the cutting means which cut the lateral areas of the web along the width thereof to form edges of the web. During edge cutting, fibers and fillers become separated from the web and they adhere to and accumulate on the edges of the steam box and drop therefrom onto the web causing runnability problems and even web breaks on some machines.
This has led to the fact that it has been necessary to place the edge cutting devices far from the headbox which, however, cannot always be done because of lack of space, and thus it may have been necessary to give up the use of the steam box when increasing drying capacity of the paper or board machine.
Attempts have also been made in the prior art to prevent the soiling problems of the steam box caused by edge cutting by means of an air curtain arranged in front of the steam box. However, this problem will be further aggravated when the running speeds of machines increase and when the aim is to provide machines that are more compact than before.